1. Field of the Invention
The invention relates to improved tie-layer adhesive blends and their use for multi-layer structures containing one or more styrene polymer layers. They are particularly useful in multi-layer structures where a styrene polymer layer is bonded to a barrier resin layer. The adhesive blends are comprised of a polyolefin base resin, a functionalized polyolefin and a combination of styrene triblock copolymers.
2. Description of the Prior Art
Multi-layer structures are widely used for food packaging applications such as for the production of thermoformed cups, trays and the like. While structures will vary depending on the intended application, styrene polymers are commonly employed as one of the layers to provide structural integrity, i.e., stiffness, and moldability. These structures also typically have an ethylene-vinyl alcohol (EVOH) or polyamide barrier resin layer. A continuing problem within the industry is how to effectively bond the dissimilar resin layers within such multi-layer constructions.
Adhesive compositions referred to as tie-layer adhesives comprised of unmodified and modified, i.e., functionalized, polyolefins and a styrenic polymer have been widely used for this purpose. While numerous tie-layer adhesives of the above types are known, there is a continuing need for adhesive compositions which provide improved adhesion to styrene polymer layers, particularly in multi-layer structures containing styrenic and barrier resin layers.
U.S. Pat. No. 5,709,953 discloses adhesive compositions comprised of 35-65 weight percent (wt. %) ethylene polymer fraction and 35-65 wt. % styrene/aliphatic/styrene triblock elastomer fraction. The reference discloses that all or a portion of the ethylene polymer fraction can be grafted with an unsaturated carboxylic acid or anhydride and, for one embodiment where an ethylene polymer fraction having relative low density is desired, suggests mixing a relatively small amount of grafted high density polyethylene (HDPE) with a relatively large amount of linear low density polyethylene (LLDPE). While the reference broadly discloses styrene/aliphatic/styrene triblock elastomers, styrene-butadiene copolymers are the only triblock copolymer used and there is no indication in the reference to the use of mixtures of styrene copolymers or that any advantage may be realized thereby.
U.S. Pat. No. 6,184,298 discloses adhesive blends containing a modified polyethylene, an unmodified polyethylene and a styrene-based elastomer. Conventional styrene-butadiene copolymers are employed for the reference compositions and triblock styrene resins are not mentioned.
Combinations of polystyrenics and functional polymers and, optionally, polyolefins and/or elastomers are disclosed in U.S. Pat. No. 6,855,432. Blends and mixtures of polystyrenics are indicated to be useful but there is no suggestion to the use of specific triblock blends or that any advantage may be achieved by such use.
Various styrene block copolymers and mixtures are disclosed in combination with amorphous tackifying resins, ethylene polymers and modified ethylene polymers in European Publication No. 0 664 327 A1.
Copending U.S. application Ser. No. 11/332,939 (US-2007-0167569-A1) discloses adhesive blends comprised of specific LLDPE copolymers, a functionalized polyethylene and styrene-isoprene-styrene (SIS) triblock copolymers. Specific SIS copolymers, i.e., containing greater than 35% styrene and less than 1% diblock, are employed to achieve the improved adhesion results reported therein.